1. Field of the Invention
The present invention relates to a mixed blue emitting phosphor, for cathode ray tube and more particularly to a mixed blue emitting phosphor having improved luminance and luminance-saturation characteristics.
2. Description of the Related Art
Along with the achievement of enlargement and high definition of color cathode ray tubes, display tubes, projections tubes, etc., various studies are being carried out to enhance the luminance of a screen. Especially, as current density is heightened, the chromaticity coordinates of light emitted from phosphors shift and luminance variation occurs. Thus, concentration is on developing phosphors which have an excellent current-to-emission characteristic (hereinafter, referred to as the .gamma.-characteristic) as well as very good temperature and durability characteristics. The phosphor which should particularly have an excellent .gamma.-characteristic is the blue emitting phosphor. This is because, while the green and red emitting phosphors utilized in the projection tube which is driven under high current density and high voltage are composed of rare earth elements and thus have an inherently good .gamma.-characteristic, the zinc sulfide (ZnS)-based phosphor used as the blue emitting phosphor has a poor .gamma.-characteristic since it does not belong to the rare earth phosphors.
In view of this problem, different from the ZnS:Ag,Cl phosphor for the use as the blue emitting phosphor in a general cathode ray tube, a ZnS:Ag,Al phosphor having an improved .gamma.-characteristic has been developed. However, there is room for further improvement of this phosphor, which thereby stipulates the need for a better phosphor.
Accordingly, in order to improve the characteristic of the ZnS:Ag,Al phosphor, research is being conducted on the control of phosphor characteristics, on the partial replacement of the host and/or activator with another element and on the use of mixed phosphors.
Japanese Patent No. sho 52-30158, and Japanese Patent Laid-open Publication No. sho 55-1003 describe that, when the ZnS:Ag,Cl and ZnS:Ag,Al phosphors have a cubic crystalline structure, they display excellent luminance characteristics.
European Patent Laid-open Publication No. EP 0 408 113 Al discloses a technique wherein chromaticity coordinates are adjusted by adding cadmium (Cd) and selenium (Se) to ZnS:Ag phosphor to thereby enhance the emission characteristic. Japanese Patent Laid-open Publication No. hei 2-255791 improves the luminance-saturation characteristic by adding Cd and Se to ZnS:Ag,Al phosphor.
Also, Japanese Patent Laid-open Publication No. hei 2-135276 discloses a technique for improving the luminance under a high current density by mixing ZnS:Ag,Al phosphor with a secondary phosphor, e.g., [(Sr,Ca,Eu).sub.3 Mg]ZnSi.sub.2 O.sub.8, Ca.sub.3 MgEu(PO.sub.4).sub.2 or (ZnMEu).sub.2 Al.sub.4 Si.sub.5 O.sub.18.
However, any luminance increase is slight when achieved by lengthening the wave of a single phosphor or mixing the phosphors. Thus, it has been determined that the effect of increasing the liminance attained by mixing the ZnS:Ag,Al phosphor (currently the best-known blue emitting phosphor for a projection tube) with another blue emitting phosphor, cannot satisfy the demand.
As described above, the ZnS-based phosphors is chiefly used as the blue emitting phosphor for a cathode ray tube. Although these phosphors have excellent luminance, they have poor durability and luminance saturation characteristics, and therefore are unfavorable for the use in projection tubes to be stimulated under high voltage and high current.
On the other hand, copper, gold-activated, aluminum-coactivated, zinc sulfide (ZnS:Cu,Au,Al), copper-activated, aluminum-coactivated zinc sulfide (ZnS:Cu,Al) phosphors are widely used as the green emitting phosphor for the cathode ray tube. However, since these ZnS-based phosphors have luminance-saturation, temperature and durability characteristics unsuitable for use in projection tubes, phosphors of rare earth elements, e.g., terbium-activated yttrium aluminum galate (Y.sub.3 (Al,Ga).sub.5 O.sub.12 :Tb), terbium-activated yttrium silicate (Y.sub.2 SiO.sub.5 :Tb) and terbium-activated lanthanum oxychloride (LaOCl:Tb), have been developed to solve the above-mentioned problem, and are currently being used satisfactorily.